The present invention relates to an electrophotographic photoreceptor having photosensitivity in the longer wavelength region and more particularly, it relates to an electrophotographic photoreceptor in which zinc oxide is used as an electroconductor and which is suitable for electrophotographic offset master adapted to exposure to semiconductor laser beams.
The conventionally used electrophotographic offset masters comprise an electroconductive support and a photoconductor layer mainly composed of photoconductive zinc oxide, a binder resin and a sensitizing dye and provided on the support.
Zinc oxide has been widely used as an electrophotographic photoreceptor because it is inexpensive, easy in handling and causes no environmental pollution. The sensitive wavelength region peculiar to zinc oxide is around ultraviolet region (380 nm) and when it is used as electrophotographic photoreceptors, in order for giving it a sensitivity to visible light, sensitizing dyes such as Rose Bengale, erythrosine, fluorescein, Methylene Blue, Crystal Violet, Bromophenol Blue and others are added thereto to carry out spectral sensitization. The conventional zinc oxide electrophotographic offset masters using these sensitizing dyes have been made by camera type direct plate making method employing a halogen lamp or the like as a light source.
Recently, scanning exposure method with laser beams is being increasingly used. This scanning exposure method has many merits as compared with conventional plate making methods since according to this scanning exposure method a direct writing in an offset master can be conducted with electrical signals from a computer and various functions such as enlargement and reduction, negative-positive conversion and screening can be imparted at the time of plate making. As the laser beam source used for this scanning exposure method, semiconductor lasers (wavelength region 700-1000 nm) which are inexpensive, small in size and make the direct conversion possible are noticed at present. However, the sensitive wavelength region of the sensitizing dyes used in the conventional zinc oxide electrophotographic offset masters is at most 650-700 nm and they do not have sensitivity to the region of 700-1000 nm and cannot be used when a semiconductor laser beam source is employed for exposure.
As electrophotographic photoreceptors applicable to exposure to a laser beam source, those which use phthalocyanine compounds are proposed in Japanese Patent Kokai (Laid-Open) Nos. 59-116760, 59-220753, 61-156132, 1-202758, 3-48856 and 3-50557. However, when these photoreceptors are used as offset masters, they suffer from the problems such as occurrence of stains in the background area, poor printing endurance and insufficient sensitivity because the photoreceptors cannot be sufficiently made insensitive to grease by etch solutions. Furthermore, use of polymethine type cyanine dyes as spectral sensitizers is disclosed in U.S. Pat. Nos. 3,619,154 and 3,682,630 and Japanese Patent Kokai (Laid-Open) Nos. 57-46245, 58-42055, 58-58554, 58-59453, 59-22053, 59-78358, 60-26949, 59-78358, 60-26949 and 62-220962. Some of them are superior in their properties, but others have the defects that when they are used for electrophotographic photoreceptors, sensitivity is insufficient and they are inferior in storage stability peculiar to polymethine dyes.